Method of producing metal powder

ABSTRACT

A method of producing metal powder, in which molten metal which is stored in a molten metal holding furnace is atomized using, a molten metal nozzle upward to generate fine liquid droplets from the molten metal and the droplets are rapidly solidified by cooling, including: preparing at least one metal melting furnace which is configured to melt metal to form molten metal, and a molten metal holding furnace which has a trough which receives the molten metal and sends the received molten metal into the molten metal holding furnace, atomizing molten metal which is stored in the molten metal holding furnace upward by a molten metal nozzle to generate fine liquid droplets of the molten metal and rapidly solidifying the droplets by cooling to produce metal powder, and controlling a molten metal level of the molten metal in the molten metal holding furnace by melting metal in the metal melting furnace to form molten metal and supplying the molten metal to the trough from the metal melting furnace.

FIELD OF THE INVENTION

The present invention relates to a method of producing metal powder.

Priority is claimed on Japanese Patent Application No. 2018-240349, filed on Dec. 21, 2018, the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION Description of Related Art

An atomizing method has been known as a method of producing metal powder. The atomizing method is a method wherein a molten metal is atomized using a molten metal nozzle and the molten metal which is atomized as fine liquid droplets is rapidly solidified by cooling. The atomizing method has been applied for producing various kinds of metal powders of aluminum, magnesium, titanium, nickel, iron, copper, tin, lead, and the like and alloys of the metals, as a method which can industrially and efficiently produce a fine metal powder having uniform particle size.

Examples of the atomizing method include a method wherein the atomizing direction of molten metal is upward, a method wherein the atomizing direction of molten metal is downward, and a method wherein the atomizing direction of molten metal is horizontal. In a case where powder of light metal such as aluminum and an aluminum alloy, which has relatively small specific gravity, is generated, the method wherein the atomizing direction of molten metal is upward is widely used. In the method wherein the atomizing direction of molten metal is upward, a molten metal nozzle is used which has a molten metal discharge port, which is provided at an upper end thereof and ejects molten metal, and a molten metal inlet port, which is provided at the lower end thereof and through which molten metal is introduced. A molten metal atomizing device which injects gas (atomizing gas) to the molten metal discharge port of the molten metal nozzle from a lower side to an upper side is attached to the molten metal nozzle to atomize molten metal. That is, the molten metal inlet port al the molten metal nozzle is immersed in molten metal which is stored in a molten metal holding furnace, and atomizing gas is injected from the lower side to the upper side toward the molten metal discharge port of the molten metal nozzle. In this way, negative pressure is generated around the molten metal discharge port, and molten metal is atomized upward from the molten metal discharge port.

Patent document 1 discloses a method of producing aluminum alloy powder using a method in which the atomizing direction of molten metal is upward. In the atomizing apparatus disclosed in Patent document 1, a molten metal inlet port of a molten metal nozzle is immersed in a molten aluminum alloy which is included a molten metal holding chamber.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2017-155270

SUMMARY OF THE INVENTION Technical Problem

In order to stably generate metal powder, which has excellent uniformity of particle size, with high production efficiency using a method in which molten metal is atomized upward, it is necessary to increase an average amount of molten metal, which is atomized from a molten metal discharge port of a molten metal nozzle, and maintain the amount to be constant (that is, fluctuation caused with the elapse of time is requested to be reduced). However, when aluminum alloy powder is generated using the atomizing apparatus disclosed in Patent document 1, a phenomenon is caused such that a molten metal level (molten metal surface height) decreases as an atomizing time elapses, a distance of a suction height of the molten metal nozzle from the molten metal level of molten metal increases, that is, a head loss increases, an atomizing amount of molten aluminum reduces, and a particle diameter of the generated powder decreases due to the reduction of the atomizing amount.

As a method of solving the above problem, a method may be considered that a variation of a molten metal level of molten metal included in a molten metal holding furnace, which is caused as atomizing time elapses, is reduced. For example, when a molten metal holding furnace which is shallow is used, the amount of a variation of the molten metal level of molten metal may be reduced. However, in this case, when a storable amount of the molten metal, which is stored in the molten metal holding furnace, is small, a step of tilting and pouring of molten metal to a molten metal holding furnace should be performed repeatedly in a short time interval to make up for consumed molten metal. In addition, since the operation is performed intermittently, a molten metal holding furnace is cooled, and it is necessary to control a temperature of molten metal, which is poured by tilting in molten metal holding furnace. Furthermore, since a furnace wall of the molten metal holding furnace is exposed to air, an oxide is generated from molten metal which remains on the furnace wall. Accordingly, a loss of raw materials is caused, and cleaning of the molten metal holding furnace is required every time before the steps of tilting and pouring molten metal. Therefore such a method is inefficient.

In addition, a method may be considered that a height position of a molten metal atomizing device (atomizing device) is controlled to be synchronized with a molten metal level of molten metal. However, in this case, there is a problem that a range of lifting and lowering is limited due to the configuration of the molten metal atomizing device. In addition, when the molten metal level is lowered, a part which does not contact with molten metal is generated at a molten metal nozzle. Therefore, a temperature in the molten metal nozzle partially decreases, and there is a case that a nozzle is closed due to solidified molten metal.

As a still another method, means may be considered that molten metal included in a molten metal holding furnace is pressed to compensate for a variation of the head loss. However, in this case, since molten metal having a high temperature is pressed, the structure of an apparatus used for thee method is made complicated, and it is difficult to design an apparatus which can achieve mass production and efficient operation.

The present invention has been made in view of aforementioned circumstances, and an object of the present invention is to provide a method of producing metal powder which uses an atomizing apparatus wherein molten metal is atomized upward, wherein metal powder having excellent uniformity of particle size can be more efficiently produced for a long period of time in succession, and productivity can be improved.

The inventors found a method wherein a molten metal level (molten metal surface height) is maintained to be constant such that a trough is provided to a molten metal holding furnace, a metal melting furnace which is used to melt metal is prepared apart from the molten metal holding furnace, molten metal which is melted by the metal melting furnace is supplied to the trough, and the molten metal supplied to the trough is sent into the molten metal holding furnace. The inventors found that, due to the above structure, even if an atomizing apparatus in which as metal is atomized upward is used, metal powder having excellent uniformity of particle size can be more efficiently produced for a long period of time in succession, and productivity of the powder can be improved, and in this way, the present invention is achieved.

That is, the present invention provides the following means to solve the aforementioned problems.

Solution to Problem

(1) A method of producing metal powder according to the first aspect is a method in which molten metal which is stored in a molten metal holding furnace is atomized using a molten metal nozzle upward to generate fine liquid droplets from the molten metal, and the liquid droplets are rapidly solidified by cooling, the method comprises

preparing at least one metal melting furnace which is configured to melt metal so as to form molten metal, and a molten metal holding furnace which has a trough which receives the molten metal and sends the received molten metal into the molten metal holding furnace,

atomizing molten metal, which is stored in the molten metal holding furnace, upward by a molten metal nozzle to generate fine liquid droplets of the molten metal, and rapidly solidifying the droplets by cooling to produce metal powder, and

controlling a molten metal level of the molten metal in the molten metal holding furnace by melting metal in the metal melting furnace to form molten metal and supplying the molten metal to the trough from the metal melting furnace.

(2) In the aforementioned aspect described in (1), the molten metal may be molten aluminum or a molten aluminum alloy.

(3) In the aforementioned aspect described in (1) or (2), a pair of the metal melting furnaces may be prepared, and when one of the metal melting furnaces supplies the molten metal, which is melted in the metal melting furnace, to the trough, the other metal melting furnace may melt metal without supplying the molten metal to the trough.

(4) In the aforementioned aspect described in any one of (1) to (3), the metal melting furnace may be controlled in such a manner that the molten metal is supplied to the trough so that a variation of molten metal included in the molten metal holding furnace is controlled in a range of ±170 mm.

Effect of Invention

According to the present invention, a method of producing metal powder can be provided which can generate metal powder, which has excellent uniformity of a particle size, in succession more efficiently for a long period of time using an atomizing apparatus which atomizes molten metal upward, and can improve the productivity of the metal powder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural view of a metal powder production system, which is usable fora method of producing metal powder according to an embodiment of the present invention.

FIG. 2 is a schematic enlarged cross-sectional view of a molten metal tank, which is usable for the method of producing metal powder according to the embodiment of the present invention.

FIG. 3 is a schematic plan view of a metal powder production apparatus, which is usable for the method of producing metal powder according to the embodiment of the present invention.

FIG. 4 is a schematic side view of the metal powder production apparatus shown in FIG. 3

FIG. 5 is a schematic partial cross-sectional side view which explains the operation of supplying molten metal, which is generated in a metal melting furnace of the metal powder production apparatus shown in FIG. 3, to a trough of the molten metal tank.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a method of producing metal powder according to an embodiment of the present invention will be described in detail with suitable reference to the drawings. The drawings used in the following description may be shown with enlarged characteristic parts for convenience's sake, for easy understanding of the characteristics of the invention, and dimensions, ratios, and the like of each constituent element may be different from actual values. In addition, the materials, the dimensions and the like shown in the following description are merely examples, the invention is not limited thereto, and these can be suitably changed within a range without changing the gist thereof. That is, the positions, the numbers, the shapes, the materials, the configurations, and the like can be added, omitted, replaced, or changed within a range without departing the gist of the invention.

FIG. 1 is a schematic structural view of a metal powder production system which is usable for a method of producing metal powder according to an embodiment of the present invention. FIG. 2 is a schematic enlarged cross-sectional view of a molten metal tank, which is usable for the method of producing metal powder according to the embodiment of the present invention. FIG. 3 is a schematic plan view of a metal powder production apparatus, which is usable for the method of producing metal powder according to the embodiment of the present invention. FIG. 4 is a schematic side view of the metal powder production apparatus shown in FIG. 3. FIG. 5 is a schematic partial cross-sectional side view which is used to explain the operation of supplying molten metal, which is generated in a metal melting furnace of the metal powder production apparatus shown in FIG. 3, to a trough of the molten metal tank.

A metal powder production system WO comprises a metal powder production apparatus 10, a metal powder recovery apparatus 50, a cyclone 60, and a metal powder recovery tank 70 as shown in FIG. 1. The metal powder production apparatus 10 produces metal powder 4. The metal powder recovery apparatus 50 is used to suck the metal powder 4, which is generated by the metal powder production apparatus 10, using a carrier airflow which is generated by an air blower (not shown). The cyclone 60 collects the metal powder 4 included in the sucked carrier airflow, and the metal powder recovery tank 70 temporarily stores the collected metal powder 4.

The metal powder production apparatus 10 comprises a molten metal tank 11, a molten metal atomizing device 20, a metal melting furnace 30, and a molten metal supply means 40 (device).

The molten metal tank 11 comprises a molten metal holding furnace 12 which stores molten metal 1, and has a trough 13 which is provided at a side of the molten metal holding furnace 12. The trough 13 receives the molten metal, and sends the received molten metal to the molten metal holding furnace 12. The molten metal holding furnace 12 preferably has a heating element which is used to maintain a temperature of molten metal to a predetermined value. As the heating element, a heat controlling device of the related art which can control a temperature of molten metal to be constant can be used, and examples thereof include a heavy oil burner, a resistance heater and an induction heater. In FIG. 1, on the surface of the molten metal 1 which is stored in the molten metal holding furnace 12, a slag layer 2 including metal oxide is generated. The slag layer 2 has a function of preventing internal oxidation of the molten metal 1.

The molten metal atomizing device 20 includes a molten metal nozzle 21 and a gas injector 25 as shown in FIG. 2. A molten metal nozzle 21 has a molten metal discharge port 22 from which molten metal 1 is ejected at an upper end thereof, and has a molten metal inlet port 23 to which molten metal 1 is introduced at a lower end thereof. The gas injector 25 has gas inlet ports 26 to which gas 3 is introduced and a gas nozzle part 27 from which the gas 3 is injected. The gas injector 25 has a gas inflow space 28 which has a cylindrical shape, and the gas inlet ports 26 are located at the positions diagonal to each other along a tangent line direction of the gas injector 25. Accordingly, a gas which is introduced into the gas inflow space 28 from the gas inlet ports 26 swirls in the gas inflow space 28 to generate a swirling flow of the gas. Introducing ports of the two gas inlet ports 26 may be arranged point symmetrically to the center of the molten metal nozzle 21. The two gas inlet ports 26 may be formed parallel to each other.

The gas nozzle part 27 injects a swirling flow of the gas 3, which flows from the lower side to the upper side, toward the molten metal discharge port 22 of the molten metal nozzle 21.

The number of the metal melting furnace 30 may be one, or two or more. Two metal melting furnaces 30 are provided preferably as shown in FIG. 3. The two metal melting furnaces, that is, a metal melting furnace 30 a and a metal melting furnace 30 b, are located so that they can supply molten metal 1 a to the molten metal holding furnace 12 through the trough 13. The two metal melting furnaces 30 a and 30 b are formed parallel to each other. The trough 13 may be branched off in accordance with the number of the metal melting furnace, and the trough 13 shown in FIG. 3 includes branch parts wherein the trough 13 branches into two parts. In a case where a pair of the metal melting furnaces, that is, the metal melting furnace 30 a and the metal melting furnace 30 b are provided, for example, metal solid may be melted using the metal melting furnace 30 b which is one of the metal melting furnaces to generate molten metal 1 a, while supplying molten metal la generated by the metal melting furnace 30 a to the trough 13 of the molten metal tank 11 from the metal melting furnace 30 a which is the other one of the metal melting furnaces. By using a pair of the metal melting furnaces, that is, the metal melting furnace 30 a and the metal melting furnace 30 b, and supplying molten metal 1 a generated by the metal melting furnaces in turn, it is possible to atomize molten metal 1 included in the molten metal holding furnace 12 in succession such that a molten metal level of molten metal 1 is controlled to be constant. The metal melting furnaces 30 a and 30 b preferably have a heating means (heating device) which is used to melt metal such as sold metal which is provided in each metal melting furnace. As the heating means, an apparatus of the related art which can be used as a heating furnace to melt metal can be used, and examples thereof include a heavy oil burner, a resistance heater, and an induction heater. A temperature used for melting metal can be optionally selected in accordance with the kinds of metal.

The metal melting furnaces 30 a and 30 b are preferably structured such that a gas bubbling treatment is performed for molten metal 1 a melted in the furnaces in order to remove hydrogen gas and oxide by floatation and supply purified molten metal 1 a.

Molten metal 1 a may be supplied to the molten metal holding furnace 12 in such a manner that the metal melting furnace 30 is inclined in the trough 13 side so that the molten metal 1 a in the furnace is poured to the trough 13 due to tilting. In the method of the present invention, a step of lifting the metal melting furnace 30 by an optionally selected element, and/or a step of tilting the metal melting furnace 30. The aforementioned steps may be performed at the same time.

The molten metal supply element 40 shown in the figure preferably composes a base 41, a support 42 which extends in a vertical direction with respect to the base 41, a molten metal takeout tool 44 which is fixed to an upper end of the support 42 with a rotary axis 43, an extensible arm 45 which is extendible and fixed to the base 41 and a connecting tool 46 which rotatably combine the metal melting furnace 30 and the extensible arm 45 with each other. As shown in FIG. 5, the molten metal supply element 40 supplies molten metal 1 a to the molten metal holding furnace 12 in such a manner that the metal melting furnace 30 is inclined to the trough 13 side by increasing a length of the extensible arm 45 so that molten metal 1 a is poured to the trough 13 by tilting. The height where the metal melting furnace 30 and the molten metal supply element 40 are provided can be optionally selected. Feedback control mechanism (not shown) may be incorporated in the molten metal supply element 40, and the mechanism may be configured to measure a molten metal level of molten metal 1 included in the molten metal holding furnace 12 and/or in the trough 13 and to supply molten metal 1 a to the trough 13 so that a molten metal level is maintained at a set value. Due to the mechanism, stable continuous atomization can be realized. As a measuring method of measuring a molten metal level of molten metal 1, for example, a method in which an optical molten metal level sensor is used and a method in which a weight of the molten metal holding furnace 12 is measured by a load cell or the like may be used. Either of the methods may be used, and the measuring method can be selected while taking workability, stability, equipment cost and the like into consideration.

As described above, the method of the present invention can include a measuring step of measuring a molten metal level of molten metal 1 included in the molten metal holding furnace 12 and/or in the trough 13. In addition, the method can include a step of supplying molten metal 1 a to the trough 13 when a value measured in the measuring step is lower than a predetermined value (specified value) or lower than a range in which the predetermined value is included (specified value). The measuring step may be performed continuously or intermittently. In addition, the method may include a step of stopping the supply of molten metal 1 a to the trough 13 and/or a step of not performing the supply of molten metal 1 a to the trough 13, when a value measured in the measuring step is larger than the predetermined value or range. When the number of the metal melting furnaces 30 is two or more, a step of melting a metal by one of the metal melting furnaces 30 without supplying the molten metal may be included.

It is preferable that a variation of a molten metal level of molten metal in the molten metal holding furnace 12 is controlled so that a stag layer 2 is not swallowed up in molten metal 1 due to the variation of the molten metal level of molten metal 1 included in the molten metal holding furnace 12, since such a swallow may be caused when the molten metal 1 a is poured by tilting. As a method of controlling the variation of the molten metal level, for example, a method can be used wherein a small amount of molten metal 1 a is poured by tilting in succession little by little. In addition, in order to prevent an occurrence of a flow by which a slag layer 2 is swallowed up, a method may be used wherein a pipe (not shown), which extends from the trough 13 downward below a molten metal surface of molten metal 1 in the molten metal holding furnace 12, may be provided so that molten metal is supplied via the pipe from a position located below the molten metal surface of molten metal 1. Furthermore, a continuous gas bubbling device (such as GBF) or a device to which a ceramic filter is attached, which are used for continuous aluminum casting, can be applied. In such a case, it is possible to provide a process of generating a high-quality powder in which the removal of inclusion and the like can be performed in succession and material defect can be reduced.

The method of producing metal powder which uses the metal powder production system 100 includes a manufacturing step of manufacturing the metal powder 4 wherein molten metal 1 which is stored in the molten metal holding furnace 12 is atomized upward by the molten metal nozzle 21 to generate fine liquid droplets from the molten metal and the droplets are rapidly solidified by cooling, and a molten metal level controlling step of controlling molten metal level of the molten metal holding furnace 12 wherein molten metal which is melted in the metal melting furnaces 30 a and 30 b is supplied to the trough 13.

The molten metal nozzle 21 is immersed such that the molten metal inlet port 23 is located in the vicinity of a bottom of the molten metal holding furnace 12. Due to the structure, it is possible to prevent the generation of involving vortexes which are caused on a molten metal surface of molten metal 1 due to the suction of molten metal 1, and as a result, suspension of molten metal 1 which is caused when a slag layer 2 is broken by the vortexes and a part of the broken slag layer is caught into molten metal 1 hardly occurs. Furthermore, it is possible to prevent clogging of the molten metal nozzle 21 which is caused when slag is caught in molten metal 1 and sucked up by the molten metal nozzle 21 as well as molten metal 1, and it is possible to prevent quality deterioration which is caused when slag as a contaminant is mixed in the metal powder 4 generated. The distance between the molten metal inlet port 23 of the molten metal nozzle 21 and the molten metal surface of molten metal 1 is preferably 100 mm or more in order to prevent the molten metal nozzle 21 from sucking caused by slag. When an operation such as composition change of a target metal powder, suspension of operation or the like is performed, it is preferable that whole amount of molten metal 1 in the molten metal holding furnace 12 is sucked up using the molten metal nozzle 21. From the view point of favorability, it is appropriate that the molten metal inlet port 23 is located in the vicinity of a bottom of the molten metal holding furnace 12, and more specifically, is located at a position which is 100 mm or less from the bottom.

In the producing step of the metal powder 4, a swirling flow (atomizing gas) of the gas 3 is injected toward the molten metal discharge port 22 of the molten metal nozzle 21 from the gas nozzle part 27 of the gas injector 25. Due to the swirling flow of the gas 3, negative pressure is generated around the molten metal discharge port 22 of the molten metal nozzle 21. Due to the negative pressure, molten metal 1 included in the molten metal holding furnace 12 is sucked up from the molten metal inlet port 23 of the molten metal nozzle 21, and atomized upward from the molten metal discharge port 22. The atomized molten metal is rapidly solidified by cooling by the swirling flow of the gas 3 to generate the metal powder 4.

Using the metal powder recovery apparatus 50, the metal powder 4 is sucked up by a carrying flow which is generated by an air blower (not shown) and sent to the cyclone 60. The metal powder 4 included in the sent swirling flow is collected by the cyclone 60, and is temporarily stored in the metal powder recovery tank 70.

In the molten metal level controlling step, molten metal 1 a included in the metal melting furnace 30 a and/or 30 b is supplied to the trough 13. The trough 13 receives the molten metal 1 a which is supplied to, and then sends the molten metal 1 a, which is transferred to the trough 13, to the molten metal holding furnace 12. It is preferable that molten metal 1 a is supplied to the trough 13 so that the molten metal level of molten metal 1 in the molten metal holding furnace 12 is controlled to be constant. Due to such a supply, a variation of an atomizing amount of molten metal 1 atomized from the molten metal discharge port 22 of the molten metal nozzle 21 can be surely suppressed. Accordingly, a constant atomizing amount can be maintained, and metal powder having excellent uniformity of particle size can be more efficiently and stably produced. In addition, the slag layer 2 is hardly swallowed up in the molten metal 1 by the variation of the molten metal level, and it is possible to supply clean molten metal 1 a to the molten metal holding furnace 12.

The molten metal level of molten metal 1 included in the molten metal holding furnace 12 at the time of generating the metal powder is varied in accordance with conditions such as the sizes of the molten metal tank 11 and the molten metal nozzle 21, the composition of molten metal, and the like. It is preferable that the molten metal level (height of molten metal surface) is controlled such that a variation of the molten metal level is ±170 mm or less, more preferably ±100 mm or less, and still further preferably ±50 mm or less. In such a case, a variation of a central particle diameter of the generated metal powder can be limited to ±5 μm or less.

In the method of producing metal powder according to the present embodiment, a molten metal level of molten metal 1 in the molten metal holding furnace 12 at the time of producing the metal powder can be maintained to be constant by the molten metal level controlling step. Due to the method, it is possible to maintain an atomizing amount of molten metal 1 atomized from the molten metal discharge port 22 of the molten metal nozzle 12. Accordingly, clue to the method of producing metal powder according to the present embodiment, it is possible to generate metal powder having excellent uniformity of a particle size in succession more efficiently for a long period of time, and productivity of the powder can be improved.

The preferable embodiment according to the present invention has been described above. However, the present invention is not limited to the embodiment, and the embodiment can be optionally changed in so far as the effects of the present invention are obtained.

For example, in the present embodiment, an explanation is performed such that powder of aluminum or an aluminum alloy is produced as metal powder which is an object to be produced. However, the metal powder is not limited to them. For example, the method of the present invention can be used for forming metal powder of aluminum, magnesium, titanium, nickel, iron, copper, tin, lead, and the like and alloys of the metals. The method of producing metal powder according to the present embodiment can be used as a method of producing powder of light metal (density: 4.5 g/cm³ or less) wherein the atomizing direction of molten metal thereof is upward. Examples of light metal include magnesium and titanium.

EXAMPLES Comparative Example 1

A metal powder production apparatus 10 as shown in FIGS. 1 to 5 was prepared. The size of a molten metal holding furnace 12 of a molten metal tank 11 was set to a diameter of 390 mm and a height of 610 mm. An aluminum alloy (composition: Al—Si—Fe type) was supplied to the molten metal holding furnace 12, and the aluminum alloy was heated and melted to generate molten aluminum alloy 60L. In this case, the aluminum alloy was not supplied to two metal melting furnaces, that is, metal melting furnaces 30 a and 30 b. Next, a molten metal inlet port 23 of a molten metal nozzle 21 was immersed in the molten aluminum alloy included in the molten metal holding furnace 12. The molten metal inlet port 23 was −530 mm away from a molten metal surface of the molten aluminum alloy, that is, a distance between the molten metal inlet port 23 and the molten metal surface was set to 530 mm. Then, air was supplied to a gas inlet port 26 of a gas injector 25 to inject a swirling flow of the air from a gas nozzle part 27 at a flow rate of 2500 L/min, and the molten aluminum alloy was atomized from the molten metal discharge port 22 of the molten metal nozzle 21. In this way, an aluminum alloy powder was produced.

The following methods were performed to measure a molten metal level, an atomized amount of molten metal atomized from the molten metal nozzle, and a central particle diameter of the obtained aluminum alloy powder according to production time of aluminum alloy powder described in Table 1 below. The results thereof were shown in Table 1.

Molten Metal Level

An optical molten metal level sensor was used for the measurement. Here, the measurement of a molten metal level was performed such that a molten metal level which was measured before the production of aluminum alloy powder was started was considered as 0 mm.

Flow Amount of Molten Metal

According to the production time described in Table 1 below, aluminum alloy powder generated in a minute was separated. The amount of the separated aluminum alloy powder was calculated as an atomized amount of the molten aluminum alloy, that is, as an atomizing amount of the molten aluminum alloy which was atomized per minute.

Central Particle Diameter

Particle size distribution of the aluminum alloy powder which was separated at the time of measuring the atomizing amount of the molten metal was measured using a laser diffraction type particle size distribution measuring apparatus. The central particle diameter (D50) was obtained based on the obtained particle size distribution.

TABLE 1 Production time of Molten Atomizing amount Central particle aluminum alloy metal level of molten metal diameter D50 powder (min) [mm] [kg/min] [μm] 0 0 2.722 61 10 −130 2.395 58 20 −220 2.210 55 30 −300 2.045 53 40 −380 1.810 51 50 −460 1.619 49 62 −540 1.427 47

As shown in the results described in Table 1, the molten metal level of the molten metal holding furnace 12 was lowered as the time used for producing the aluminum alloy powder was longer. It is confirmed that, due to lowering of the molten metal level, the molten metal atomizing amount of the molten aluminum alloy was decreased and the size of the obtained aluminum alloy powder was decreased.

Example 1

An aluminum alloy was provided in each of two metal melting furnaces 30 a and 30 b and in a molten metal holding furnace 12 of a molten metal tank 11 of a metal powder production apparatus 10 which was used in Comparative Example 1. Then, an aluminum alloy provided in the molten metal holding furnace 12 and an aluminum alloy provided in the metal melting furnace 30 a, which is one of the metal melting furnaces, were heated and melted to generate the molten aluminum alloy. Subsequently, similar to Comparative Example 1, a molten metal inlet port 23 of a molten metal nozzle 21 was immersed in the molten aluminum alloy in the molten metal holding furnace 12. Then, a swirling flow of air was injected from a gas nozzle part 27 of the gas injector 25, and the molten aluminum alloy was atomized from the molten metal discharge port 22 of the molten metal nozzle 21. In this way, aluminum alloy powder was produced. In the production of the aluminum alloy powder, when the molten metal level of the molted aluminum alloy in the molten metal holding furnace 12 reached −20 mm from the molten metal level which was measured before the production of the aluminum alloy powder was started, the molten aluminum alloy included in the metal melting furnace 30 a was supplied to a trough 13 of the molten metal tank 11. The molten metal level of the molten aluminum alloy in the molten metal holding furnace 12 was controlled such that a distance between the molten metal level of the molten aluminum alloy in the molten metal holding furnace 12 and the height of the molten metal surface which was measured before start of atomizing the aluminum alloy powder is ±20 mm or less. While supplying the molten aluminum alloy included in the metal inching furnace 30 a, a molten aluminum alloy was prepared by heating in the metal melting furnace 30 b. After the molted aluminum alloy included in the metal melting furnace 30 a was consumed, changeover of the metal melting furnace from the metal melting furnace 30 a to the metal melting furnace 30 b was performed. In this way, the molten aluminum alloy in the metal melting furnace 30 b was supplied and the molten metal level of the molten aluminum alloy in the molten metal holding furnace 12 was controlled.

Due to the method described above, the aluminum alloy powder was produced in succession for eight hours. Particle size distribution of the aluminum alloy powder, which was collected when two hours elapsed since the production was started, was measured. The results shows that the average particle diameter thereof was 60 μm, and the diameter was the same as that of Comparative Example 1 which was measured immediately after the production of the aluminum alloy powder was started. Based on the result, it is confirmed that the aluminum alloy powder having excellent uniformity of particle size was produced in succession in Examples 1 wherein replenishing of the molten aluminum alloy to the molten metal holding furnace 12 of the molten metal tank 11 was performed.

INDUSTRIAL APPLICABILITY

A method of producing metal powder of the present invention has excellent effects as means for improving productivity, since a variation of the height of molten metal head causes adverse effects, and such a variation is associated with the speed of the molten metal suction which is caused due to the negative pressure generated by the swirling flow. In particular, from the view point of the industrial production scale, there is a strong desire for improving production efficiency of aluminum powder and alloy powder thereof. If productivity thereof is improved while high quality and the generation at a low cost are achieved, such a method can contribute to expansion of market.

The present invention can provide a method which can generate metal powder having excellent uniformity of a particle size for a long period of time in succession.

EXPLANATION OF REFERENCES

1 Molten metal

2 Slag layer

3 Gas

4 Metal powder

10 Metal powder production apparatus

11 Molten metal tank,

12 Molten metal holding furnace

13 Trough

20 Molten metal atomizing device

21 Molten metal nozzle

22 Molten metal discharge port

23 Molten metal inlet port

25 Gas injector

26 Gas inlet port

27 Gas nozzle part

28 Gas inflow space

30, 30 a, 30 b Metal melting furnace

40 Molten metal supply element

41 Base

42 Support

43 Rotary axis

44 Molten metal takeout tool

45 Extensible arm

46 Connecting tool

50 Metal powder recovery apparatus

60 Cyclone

70 Metal powder recovery tank

100 Metal powder production system 

1. A method of producing metal powder, in which molten metal which is stored in a molten metal holding furnace is atomized using a molten metal nozzle upward to generate fine liquid droplets from the molten metal and the liquid droplets are rapidly solidified by cooling, comprising: preparing at least one metal melting furnace which is configured to melt metal to form molten metal, and a molten metal holding furnace which has a trough which receives the molten metal and sends the received molten metal into the molten metal holding furnace; atomizing molten metal, which is stored in the molten metal holding furnace, upward by a molten metal nozzle to generate fine liquid droplets of the molten metal, and rapidly solidifying the droplets by cooling to produce metal powder; and controlling a molten metal level of the molten metal in the molten metal holding furnace by melting metal in the metal melting furnace to form molten metal and supplying the molten metal to the trough from the metal melting furnace.
 2. The method of producing metal powder according to claim 1, wherein the molten metal is molten aluminum or a molten aluminum alloy.
 3. The method of producing metal powder according to claim 1, wherein a pair of the metal melting furnaces is prepared, and when one of the metal melting furnaces supplies the molten metal, which is melted in the metal melting furnace, to the trough, the other metal melting furnace melts metal without supplying the molten metal to the trough.
 4. The method of producing metal powder according to claim 1, wherein the metal melting furnace is controlled in such a manner that the molten metal is supplied to the trough so that a variation of molten metal included in the molten metal holding furnace is controlled in a range of ±170 mm.
 5. The method of producing metal powder according to claim 1, wherein the step of controlling the molten metal level includes a measuring step of measuring the molten metal level of molten metal in the molten metal holding furnace or the trough, and a step of supplying the molten metal in the metal melting furnace to the trough when the molten metal level measured in the measuring step is lower than a specified value.
 6. The method of producing metal powder according to claim 1, wherein the step of controlling the molten metal level includes a measuring step of measuring the molten metal level of molten metal in the molten metal holding furnace or the trough, and a step of not performing supply of the molten metal in the metal melting furnace to the trough, when the molten metal level measured in the measuring step is larger than a specified value.
 7. The method of producing metal powder according to claim 2, wherein a pair of the metal melting furnaces is prepared, and when one of the metal melting furnaces supplies the molten metal, which is melted in the metal melting furnace, to the trough, the other metal melting furnace melts metal without supplying the molten metal to the trough.
 8. The method of producing metal powder according to claim 2, wherein the metal melting furnace is controlled in such a manner that the molten metal is supplied to the trough so that a variation of molten metal included in the molten metal holding furnace is controlled in a range of ±170 mm.
 9. The method of producing metal powder according to claim 3, wherein the metal melting furnace is controlled in such a manner that the molten metal is supplied to the trough so that a variation of molten metal included in the molten metal holding furnace is controlled in a range of ±170 mm.
 10. The method of producing metal powder according to claim 7, wherein the metal melting furnace is controlled in such a manner that the molten metal is supplied to the trough so that a variation of molten metal included in the molten metal holding furnace is controlled in a range of ±170 mm. 